Catalysts in tablet form are generally used for hydrogenation reactions. These tablets are notable for high mechanical stability, a property determinable in the form of the side crushing strength. The stability comes about as a result of a relatively high pressure during tableting. As a result, the powder starting material is highly compressed, producing tablets having a relatively high bulk density. The high compression also reduces the pore volume and hence restricts access to the active centers. Only part of the active metal components, therefore, are available for the reaction.
Already disclosed in the prior art are catalysts in extrudate form where the powder starting material is processed by addition of a defined amount of suitable binders and by means of extrusion to give the corresponding shaped catalyst bodies. Extrudates customarily have a greater pore volume than the tablets produced from the same powder starting material.
The catalysts in extrudate form that are known in the prior art differ from the catalysts of the invention in particular in the composition, such as the metals and types of binder used, for example, and also in the physicochemical properties.
U.S. Pat. No. 5,977,010 describes a shaped catalyst body which comprises (i) at least one metal from the group consisting of copper, manganese, zinc, nickel, cobalt, and iron, and also (ii) calcium silicate and (iii) at least one clay material. These shaped catalysts are used for hydrogenating aldehydes, ketones, carboxylic acids, and carboxylic esters.
WO 92/10290 discloses shaped bodies of a copper chromite catalyst formed from a mixture of about 20 to 80 wt % of copper chromite and 20 to 80 wt % of at least one extrudable, inorganic binder material. The catalysts have a surface area of 20 to 225 m2/g, and the total pore volume is between 0.35 and 1 cm3/g. This document describes a process for producing the shaped copper chromite catalyst by extrusion of a mixture of copper chromite, an extrudable inorganic binder material, a peptizing agent, and water, and calcining of the extrudate. The shaped bodies obtained are used for hydrogenating aldehydes, ketones, carboxylic acids, and carboxylic esters.
U.S. Pat. No. 4,666,879 describes an extruded copper chromite-aluminum oxide catalyst which is produced by mixing 40 to 82 wt % of copper chromite and 18 to 60% of an extrudable aluminum oxide, typically having a pseudoboehmite or alpha-hydroxy-boehmite structure. After being calcined, the extruded catalyst can be used for hydrogenating various carbonyl compounds in the liquid or gas phase. The catalyst has a surface area of between 20 and 225 m2/g and a bulk density of between 0.70 and 1.20 g/cm3.
WO 2006/005505 describes a process for hydrogenating organic, carbonyl-functional compounds using copper-containing catalyst tablets or catalyst extrudates having a diameter of <2.5 mm. The catalysts are produced by shaping and subsequently calcining a mixture of 50 to 80 wt % of copper oxide, 15 to 35 wt % of aluminum oxide, 2 to 20 wt % of lanthanum oxide, and copper platelets.
In the dissertation by Steffen P. Miller, University of Karlsruhe (TH), 2005, Cu/Zn catalysts in extruded form are described that are produced using boehmite-based binders.
WO 2005/058491 discloses CuO/Al2O3-containing shaped catalyst bodies in extrudate form. The catalysts are produced by mixing boehmite, which has undergone incipient etching with formic acid, with a CuO/Al2O3-containing active material and water. The mixture is then extruded to strands which are calcined at 600° C. The catalysts have a bulk density of between 790 and 960 g/l and pore volumes in the range from 0.31 to 0.59 cm3/g.
The mechanical stability of extrudates is generally lower than that of catalyst tablets, as manifested in a low side crushing strength, for example. Furthermore, the binders used in extrudates frequently have an adverse effect on catalyst performance. Major influencing parameters are the inherent activity of the binder matrix, change in the surface acidity, and diffusion effects in the matrix. These effects are described for example by K. P. de Jong in “Syntheses of solid catalysts”, 2009, Wiley-VCH, p. 175, or in the dissertation by J. Freiding: “Extrusion of technical ZSM-5 catalysts and their application in the MTO process”, University of Karlsruhe (TH), 2009, especially section 2.5 and section 5.6. In DE 10 2006 058800, therefore, shaped catalyst bodies which are extremely pure are explicitly claimed, in order to prevent adverse effects arising from the catalyst matrix.
It is an object of the present invention to provide shaped catalyst bodies which do not have the aforementioned drawbacks of catalyst tablets and catalyst extrudates, thus having a substantially higher pore volume and a significantly lower bulk density in conjunction with at least comparable stability and activity in comparison to conventional, tableted catalysts.
This object is achieved in accordance with the invention by using, when processing powder catalyst material into extrudates, specific binders which lead to stable shaped catalyst bodies. Through the processing of the powder catalyst material into extrudates using binders which lead to stable shaped bodies and at the same time generate a high pore volume, it has been possible to produce shaped catalyst bodies which, for comparable stability, have a substantially higher pore volume than the conventional, tableted catalysts.